The present invention relates to a system for the assisted selection of the ratios of a gearbox or transmission, in particular on an automobile vehicle, this gearbox being provided with an input element movable in rotation and translation or solely in rotation, for effecting the chosen selection.
Various conventional types of gearboxes are known among which may be mentioned the following: electromagnetic gearboxes equipped with an electromagnet and selecting epicyclic reduction gears, gearboxes having a pneumatic preselection used in particular on buses, transfluid gearboxes with a hydraulic coupling, hydraulically controlled gearboxes with a hydraulic directional control valve on the steering wheel.
The gearbox having a hydraulic coupling and an automatic or a semi-manual selection has taken precedence over the other structures. These gearboxes without a supplementary device have losses due to slip.
Patent GB-A-2,214,248 discloses a mechanism having controlled cylinders in which the command for achieving the required position is the resultant of a calculation carried out via an electronic control system, between an instruction the level of which is pre-established electrohydraulically and a position of rest.
The various positions to be obtained with respect to the speeds to be engaged result from the association of a proportional solenoid valve, a cylinder, a displacement sensor and an electronic device. It therefore concerns an electrohydraulic position control which has the major drawback, in such an application, of positioning the cylinder in an indeterminate manner in the event of a simple, and in particular an electric breakdown.
Semi-automatically controlled gearboxes to which the present invention relates have no hydrodynamic coupling. Their efficiency is solely that of the gear train of a traditional gearbox. The selection is effected by one of the following means:
1) by action on the shifting forks (linear selection), PA1 2) by action on a rotary barrel, PA1 3) by action on a linkage (rotolinear system).
In respect of these three known selecting modes, the actuation is hydraulic. But these devices present various drawbacks. Thus, the linear selection by action on the forks (the aforementioned selection 1)) requires many solenoid valves, namely two solenoid valves per cylinder, or a permanent return force which is hydraulic (application of the pressure on a differential section) or mechanical (spring); these other arrangements result in an increased dimension of the solenoid valves (doubling the actuating forces). The safety of the gearbox depends on the reliability of the system employing positive inhibitions which makes it impossible to engage two ratios at the same time. This solution is therefore both complex and expensive.
Actuation by a rotary barrel provides the selection and the positive inhibitions but requires a mechanical element for converting the rotary motion into linear motion which may be difficult to produce.
Depending on the architecture of the gearbox and the number of the speeds, the forks are shifted by one, two or three shafts.